1. Field of the Invention
The present invention related to alkaline storage batteries having a silver positive electrode.
In particular, the invention relates to new technology relating to silver positive electrodes, the application thereof to alkaline storage batteries, in particular in combination with a zinc negative electrode, the separators and electrolyte used, and the functioning of the silver-zinc (AgZn) storage battery thus formed, both in an open mode and in a tight mode.
2. Description of the Related Art
Electrochemical couples which use a silver electrode (silver-zinc, silver-cadmium, . . . ) have been known the 19th century.
The effective use of alkaline secondary systems having a silver positive electrode has only really been developed since 1940, after Henri Georges ANDRÉ developed a silver-zinc storage battery using cellophane separators as semi-permeable membranes and zinc electrodes, it being attempted to develop the porosity of said electrodes.
Despite this progress and other developments resulting therefrom, the service life of AgZn storage batteries has remained somewhat limited (approximately after tens of cycles), it being difficult to obtain one hundred cycles. Furthermore, they could only be used “commercially” in open configurations. This led to their application being viewed as limited to some military uses, for which above all the high levels of power specific to the system are sought.
The AgZn storage battery's low aptitude for cycling is mainly attributed to the behaviour of the zinc electrode in an alkaline environment. The behaviour of the silver electrode is also responsible for this.
The reactions which take place at the anode are as follows for an alkaline storage battery:
chargeZn+2OH−⇄ZnO+H2O+2e−with ZnO+H2O+2OH−⇄[Zn(OH)4]2−discharge
In fact, the zinc electrode is generally recharged with its oxides and hydroxides and zincates due to the formation of deposits which are of a modified structure compared with their original form and are often described as dendritic, spongy or pulverulent. The phenomenon occurs increasingly within a wide range of current densities.
The subsequent recharges thus lead quickly to anarchic increases or thrusts of zinc through the separators and to short-circuiting with the electrodes having opposite polarity.
The pulverulent or spongy-type deposits do not allow electrodes which are able to function in a satisfactory and lasting manner to be restored since the adhesion of the active matter is insufficient.
Moreover, the reduction of oxides, hydroxides and zincates to metal zinc at the anode during recharging phases is also characterised by changes to the morphology of the electrode itself. In accordance with the operating modes of storage batteries, different types of modifications to the shape of the anode are observed, caused by the zinc being redistributed in a non-uniform manner during its formation. In particular, this may result in situ in a detrimental densification of the anodic active mass at the surface of the electrode, most likely at the central region thereof. At the same time, the porosity of the electrode is generally reduced, which helps accelerate the preferential formation of zinc at the surface of the electrode.
These significant drawbacks which reduce the number of cycles that can be carried out to only a few tens (a level which is insufficient to afford a secondary system any real economic benefit) have led to a vast number of studies being carried out with the objective of improving the features of the zinc deposit during recharging with a view to increasing the number of charge-discharge cycles which the generator could support.
A substantial development was made and disclosed in the disclosure of invention of publication number FR 2 788 887, supplemented by that of publication number FR 2 828 335, the technology described of zinc anodes making it possible to achieve hundreds of cycles within a wide range of operating modes and up to very high discharge levels owing to the use of means for increasing the performance of the active matter used by improving the percolation of the charges in said active matter.